Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
It will look like the heading of a section because of the sizing of the text and because it is grouped in a box. Other alerts that are not dismissible and have the exact same appearance do use a heading. Also why wouldn't we want an alert to be conveyed in the document outline? If it is important enough to warrant an alert then why is it not important enough to include it in the outline?